Aluminum is produced in Hall-Heroult cells by the electrolysis of alumina in molten cryolite, using conductive carbon electrodes. During the reaction the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction ##EQU1##
The problems caused by consumption of the anode carbon are related to the cost of the anode consumed in the reaction above and to the impurities introduced to the melt from the carbon source. The petroleum cokes used in the anodes generally have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing particularly troublesome workplace and environmental pollution. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon is consumed in the reduction the overall reaction would be 2Al.sub.2 O.sub.3 .fwdarw.4Al+3O.sub.2 and the oxygen produced could theoretically be recovered, but more importantly no carbon would be consumed at the anode and no contamination of the atmosphere or the product would occur from the impurities present in the coke.
Attempts have been made in the past to use non-consumable anodes with little apparent success. Metals either melt at the temperature of operation, or are attacked by oxygen or by the cryolite bath. Ceramic compounds such as oxides with perovskite and spinel crystal structures usually have too high electrical resistance or are attacked by the cryolite bath.
One of the problems arising in the developing of conductive ceramic anodes has been caused by the difficulty of making a durable electrical connection between the anode and the current conductor. Previous efforts in the field have produced connectors, primarily of metals such as silver, copper, and stainless steel. Can, U.S. Pat. No. 3,681,506, disclose a resilient metal washer held in place to form an electrical connection. Davies, U.S. Pat. No. 3,893,821, disclose a contact material containing Ag, La, SrCrO.sub.3 and CdO. Douglas et al., U.S. Pat. No. 3,922,236, disclose a contact material containing Ag, Cu, La, and SrCrO.sub.3. Fletcher, U.S. Pat. No. 3,990,860, disclose cermet compositions containing stainless steel or Mo in a matrix of Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3. Shida et al., U.S. Pat. No. 4,141,727, disclose contacts of Ag, Bi.sub.2 O.sub.3, SnO.sub.2 and Sn. Schirnig et al., U.S. Pat. No. 4,247,381, disclose an electrode useful for AlCl.sub.3 electrolysis comprising a graphite pipe, a metallic conductor with a melting point below the bath temperature, and a protective ceramic pipe surrounding the former. West German No. 1,244,343, U.S. Ser. No. 729,621, discloses borides or carbides of Ti, Zr, Ta, or Nb cast of Al using a flux of Li.sub.3 AlF.sub.6, Na.sub.3 AlF.sub.6 and NaCl. Alder, U.S. Pat. No. 4,357,226, discloses an anode assembly for a Hall cell comprising individual units mechanically held together by a clamping arrangement.
There have been several lines of development concerning nonconsumable anodes, with ceramics such as stannic oxide compounds, spinels, perovskites and various cermets as principal materials under study. A cermet is a composite material containing both metal and ceramic phases. All of these need some method for connecting to the current conductor. Landon et al., U.S. Pat. No. 4,462,889, disclose a cermet composition for a non-consumable electrode of this type. Secrist et al., U.S. Pat. No. 4,472,258, disclose a non-consumable electrode with a gradient cermet composition. Secrist et al., U.S. Pat. No. 4,484,997, disclose a non-consumable anode with a diffusion-limited composition. Clark et al., U.S. Pat. No. 4,491,510, disclose a non-consumable electrode having a specific configuration. Secrist et al., U.S. Pat. No. 4,495,049, disclose an electrode with a specific gradient metal-cermet joint.